The present invention relates to a method for power generation, and more specifically to a power generation method that utilizes a portion of a stream of oxidized gas produced by a pollution abatement process to generate power without generating any additional carbon dioxide.
The Clean Air Act of 1970 and 1990 imposed upon the country the need to control the emission of Volatile Organic Compounds (VOCs) and Hazardous Air Pollutants (HAPs) to the atmosphere. VOCs and HAPs are found in significant amounts in waste gas streams created as a result of the implementation of industrial processes. Since VOCs are a precursor of smog, and HAPs are typically detrimental to health, the amount of VOCs and HAPs that are released into the atmosphere need to be substantially reduced or eliminated entirely.
The industries and processes that need to control their output of VOCs and HAPs include the printing, chemical, pharmaceutical manufacturing, automotive coating and painting, bakeries, can coating, wood manufacturing, medical device sterilization, soil remediation, and metal decorating industries, among others. The gas flow volumes output by these various types of operations can vary from between 100 CFM and 100,000 CFM. The VOCs and HAPs in the output gas streams are measured in terms of the LEL (lower explosive limit), where 100% of the LEL, which is different for each organic compound, means that a spark in the presence of a vapor of the organic compound at 100% LEL will yield an explosion. Since all companies need to avoid explosions, the National Fire Protection Association, NFPA, has required that the amount of the various compounds in the waste gas stream should be below 25% of the LEL, which in turn gives a 4:1 safety factor, if no LEL measurement is made, and below 50% of the LEL if continuous measurements are made.
The waste or process gas streams actually being produced that need to have their pollutant levels controlled have much lower organic compound levels and percent LEL, and represent significantly higher air volumes than those treated in the past. Specifically, most waste gas streams produced today contain from about 5% of the LEL to about 1% LEL. However, due to increasingly stringent restrictions on the amounts of VOCs and HAPs that can be discharged to the atmosphere, even these smaller overall amounts of organic compounds must be removed from waste gas streams. Thus, the waste process gas streams must be passed through facilities that can eliminate the VOCs and HAPs from the streams.
In most cases, the VOCs and/or HAPs are removed from the gas stream by oxidizing the VOCs or HAPs in the stream. Simply put, oxidization is the reaction of an organic compound with an oxidizing agent, such as a catalyst or oxygen. There are two fundamental methods for oxidizing a hydrocarbon such as a VOC or a HAP. One method of performing oxidation of hydrocarbons such as VOCs and HAPs in a gas stream is the thermal oxidization method. Thermal oxidation is a method where a hydrocarbon molecule (hydrogen+carbon) such as a VOC or HAP is raised to a temperature where the hydrocarbon in the waste gas stream reacts with oxygen that is present or added to the waste gas stream to form carbon dioxide (CO2) and water vapor (H2O) plus heat, where the energy given off by the oxidation reaction is due to the combustion of the VOCs and/or HAPs which are being oxidized. More particularly, in an oxidation reaction the hydrocarbon (VOC or HAP) is raised to oxidation temperatures of between 1400-1800 degrees F. at which the oxidation reaction can occur rapidly, and held at this temperature for a specified xe2x80x9cresidence timexe2x80x9d of from 0.5 seconds to 2.0 seconds to ensure completion of the reaction. In addition to temperature and time, a third variable that determines the efficacy of the oxidation method is turbulence to ensure sufficient contact of the oxidation reactants with one another, which is achieved by the design of the equipment through which the waste gas stream flows during the thermal oxidization process.
A second method of oxidizing a hydrocarbon is the catalytic oxidation method. In this method, a catalyst, similar to the catalyst in an automobile catalytic converter, reacts with the hydrocarbons (VOCs and HAPS) in the gas stream passing through the catalyst to convert the hydrocarbons to the same reaction products as for thermal oxidation, namely, carbon dioxide (CO2) and water vapor (H2O) plus heat. The catalyst allows the oxidation process to take place at significantly lower temperatures, i.e., from 450-800 degrees F. Also, in the catalytic oxidation method the residence time of the process gas stream in the equipment is reduced to 0.1-0.3 seconds. However, due to the cost of the catalyst used in this method and the inability to regenerate the catalyst, the catalytic oxidation method is frequently cost prohibitive for use in conjunction with large industrial process gas streams. Thus, thermal oxidation is the preferred method for eliminating VOCs and HAPs from gas streams.
When the thermal oxidation method is to be employed, there are three types of thermal oxidizers that can be used. The three types of thermal oxidizers are: (1) a direct fired oxidizer; (2) a recuperative thermal oxidizer; and (3) a regenerative thermal oxidizer.
The direct fixed oxidizer operates on the principal that the combustion process gas stream is brought into a furnace section of the direct oxidizer, in which the temperature of the gas stream is raised to 1400 degrees F. The process gas stream is held in the furnace section at this temperature for the required residence time in order to fully oxidize the VOCs and HAPs in the stream, as discussed above. However, a significant problem with direct fired oxidizers when they are used to oxidize hydrocarbons in a process gas stream is that the incoming process gas stream is frequently not constant in terms of flow rate or in terms of the available BTUs from the varying amounts of hydrocarbon components in the process gas stream that are to be oxidized. Therefore, because the heat output of the combustion of the hydrocarbon in the process gas stream is alone not sufficient to raise the temperature of the incoming process gas flow to above the necessary oxidation temperature, the energy supplied by the hydrocarbons to raise the temperature of the process gas stream will often have to be supplemented with heat supplied by burning a large amount of natural gas or oil. As a result, the use of a direct fixed oxidizer is not cost effective because of the high-energy input to raise and maintain the temperatures of the VOC laden gases from ambient temperatures to above at least 1400 degrees F.
The second type of thermal oxidizer is the recuperative thermal oxidizer. This type of oxidizer operates on the principal that in order to reduce the energy input necessary for destroying the VOCs and HAPs in the process gas stream. To do so, a metallic recuperator is positioned directly upstream of the oxidizing chamber and is used to preheat the incoming process gas stream using the previously oxidized gas stream which has been raised to above 1400 degrees F, thereby reducing the amount of supplemental fuel required to bring the incoming process gas stream up to the oxidizing temperature. The metallic recuperators used in a recuperative thermal oxidizer can achieve an efficiency of 70-80% when used in an oxidation process.
The third and final type of thermal oxidizer is the regenerative thermal oxidizer. This type of oxidizer is specifically designed for use in oxidizing large process gas flows having low organic compound concentrations, i.e., low percentages of VOCs and HAPs in the process gas stream. This type of oxidizer includes an oxidizing chamber that is connected to a number of ceramic heat exchangers which are used to preheat the incoming process gas stream, thereby reducing the amount of auxiliary fuel required to bring the waste gas stream up to the oxidizing temperature, similar to the recuperative thermal oxidizer. The ceramic heat exchangers are heated to preheat the incoming process gas stream by cycling the oxidized process gas stream in opposite directions through each of the heat exchangers by the operation of a valve system connected to the oxidizer. The ceramic heat exchangers used in a regenerative thermal oxidizer can achieve an efficiency of 95%.
The fuel cost of raising these large process gas streams from a low incoming temperature, which is frequently ambient, to the thermal destruction temperature of 1,400 degrees F. is quite large. Even when highly efficient heat exchangers are used to preheat the incoming gas, these fuel costs are only reduced to a certain extent, and in many cases cannot be eliminated entirely. Therefore, other avenues to reduce the overall cost of oxidizing the pollutants in these gas streams have been explored.
One possible solution is the use of distributed power generation (DPG) at the site of the oxidation process. DPG is the integrated or stand-alone use of small modular electric generation close to the point of consumption for the benefit or use of the consuming site. DPG is increasing in importance as a result of four independent trendsxe2x80x94utility industry restructuring, increasing system capacity needs, decisions to not build conventional power plants or transmission systems, and technology advancement.
In the particular situation of oxidizing pollutants in process gas streams, which must be accomplished in order to meet the present emission standards, the pollutant-free process gas stream exiting the oxidizer can be used for the cogeneration of steam and power without creating additional CO2 that would be created if the power was obtained from a conventional utility, such as a coal based power plant. The generation of power without any consequent generation of CO2 is also highly desirable based on the significant restrictions that have recently been placed on industries in many countries.
Unfortunately, prior art attempts to generate power utilizing the energy in a process gas stream exiting an oxidizer have been unable to effectively generate power without also creating additional CO2. More specifically, once the large volume of incoming process gas passes through the recuperator or heat exchanger in the process gas stream path to the oxidizer, the temperature of the incoming gas stream has been raised close to or above the auto-ignition temperature for the organic compounds in the process gas stream, and hence the VOCs and or HAPs are ignited and destroyed, releasing energy. Since the primary objective of the oxidizer (of whatever type) is to destroy the organic pollutants in the process gas stream, it is necessary to maintain the gas at or above the combustion temperature for the appropriate residence time to ensure the destruction of all VOCs and HAPs, as stated previously. Therefore, any attempt to divert the process gas stream from the oxidizer for other purposes, such as steam generation and/or power generation, will result in a reduced residence time for the process gas stream in the oxidizer and hence the inability to completely remove the pollutants and meet the EPA mandated destruction efficiencies. Even if the process gas stream were diverted after having all of the pollutants oxidized, in a recuperative or regenerative oxidizer the oxidized process gas stream would not be available to pass through the heat exchanger of the oxidizer and preheat the incoming process gas stream, thus increasing fuel usage to heat the incoming gas thereby increasing the cost of oxidizing the gas stream. As a result, these two types of oxidizers have been tied into boilers to generate the steam using the oxidized process gas stream, but only after the oxidized gas stream has passed through the recuperator or heat exchanger and lost the majority of the heat energy of the stream. Further, direct fired oxidizers, while able to divert the entire oxidized process gas stream to a boiler for steam generation, have such high energy costs, as discussed previously, as to make them unsuitable for use in distributed power generation.
Therefore, due to the high energy content of the oxidized or treated process gas stream flowing from an oxidizer, it is desirable to develop an apparatus and method for using the treated gas stream for generating power onsite. It is desirable to develop such a system with known components that does not create any CO2 in addition to that generated by the oxidizer and that does not detrimentally affect the operation of the recuperators or heat exchangers used in the oxidizer.
It is an object of the present invention to provide a system and method for the generation of on-site electric power by the diversion of a portion of the oxidized or treated process gas stream from an oxidizer that is used to remove hazardous components from the incoming process gas stream without detrimentally affecting the operation of the oxidizer.
It is a further object of the present invention to provide a system and method in which the diversion of the portion of the treated gas stream also generates steam which can be utilized in addition to the electric power.
It is another object of the present invention to provide a system and method in which the generation of the electric power is performed without the creation of any carbon dioxide (CO2) in addition to that generated by the oxidizer.
It is still another object of the present invention to provide a system and method in which the components of the system and their operation in the method are well known and easily utilized to form the system and perform the method.
The present invention is a system and method for use with equipment for oxidizing an incoming process gas stream to remove hazardous air pollutants (HAPs) and volatile organic compounds (VOCs) from the process gas stream. After the process gas stream has been oxidized or treated, the system diverts only a portion of the oxidized process gas stream through a heat exchanger and utilizes the elevated temperature of the diverted process gas stream to generate a pressurized flow of steam. The remainder of the oxidized process gas stream that is not diverted passes through a heat exchanger on the oxidizer that is used to preheat the incoming process gas stream to the oxidizer. The diverted process gas stream and the remainder of the oxidized process gas stream are then vented to the atmosphere after generating the steam, and preheating the incoming process gas stream, respectively. The steam created by the diverted process gas stream is directed to a microsteam turbine which uses the flow of pressurized steam through the turbine to rotate turbine blades fixed to a rotatable output shaft extending outwardly from the turbine. The output shaft is connected to an electric generator such that the rotation of the output shaft operates the generator to generate electric power. This electric power is then transferred to an electric switchgear to distribute the generated electric power back to the plant for use as necessary.
Further, after the now-depressurized flow of steam has exited the turbine, some or all of the flow of steam can be diverted through a plant steam header to a process boiler for further use within the plant. The remainder of the flow of depressurized steam, if any, is passed through a condenser and combined with make-up water in a feed-water heater prior to re-entering the heat exchanger for reuse in forming the pressurized flow of steam by the diverted portion of the process gas stream.
All of the power generated by the system and method of the present invention, whether by the turbine or by the steam directed to the process boiler, is created without the burning of additional fuel and the generation of additional carbon dioxide (CO2), thus making the system and method very efficient with regard to the energy output from the system.
Various other features, objects and advantages of the invention will be made apparent from the following detailed description taken together with the drawings.